Getting serial console to respond in Arch/ARM

[tawalker]

I recently took delivery of a USB-to-RS232 converter module, which I ordered from eBay UK (it was reported on this forum as working with the RasPi). My main aim is to set up a USB-serial console port, which I can use to log into the Pi from another computer - for instance, if the USB and/or Ethernet on the Pi aren't responding and I "need" to access the machine for diagnostics, etc.

Following various forum and blog posts strewn around the interwebs (mostly around this site), I connected up the module to the appropriate GPIO pins on the Pi, and used a USB extension cable to hook up the module to my netbook (running Arch/x86). My Pi is running the "hard float" version of Arch; it's a recent install, and I am still installing and setting up things there, so not everything is yet arranged how I'd ultimately like it.

When I started minicom on the netbook (115800, 8-N-1), I expected to see a login prompt from the Pi, but the terminal display stayed more or less blank, aside from the odd garbled character or three (presumably stray voltages or something?).

I am reasonably certain that the USB-serial module is connected and working correctly on the Pi. When I press keys on the netbook when minicom is running, the "RX" LED on the module flashes, and I can echo text from the Pi to its serial port (/dev/ttyAMA0) and the text appears in the minicom terminal on the netbook.

My suspicion is that the Pi's console port is either not set up somehow, or it is not directing the console I/O to /dev/ttyAMA0 properly, so minicom on the netbook doesn't have anything on the Pi to "talk to". I don't know if this is correct, but I've made sure that

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console=ttyAMA0,115200

is present in /boot/cmdline.txt.

In short: I'd be most grateful if someone could suggest what else I need to do, in order to be able to get serial console access running on my Pi?

Many thanks in advance

[pepedog]

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systemctl start getty@ttyUSBx.service

maybe?
Look see what device is in /dev folder
If you need help starting on boot, or speed problems, post back

[tawalker]

Code: Select all

systemctl start getty@ttyUSBx.service

maybe?
Look see what device is in /dev folder
If you need help starting on boot, or speed problems, post back

Thanks pepedog - I'll take a look at this and come back here with what transpires

[pepedog]

It might be ttySx where x is a number. Plugging is and looking at dmesg might help identify

[tawalker]

Thanks pepedog I think the systemd service was the key. I ran

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systemctl start getty@ttyAMA0

, and was then able to log on via minicom on the netbook.

The only small issue I've had since it started working, was that the console doesn't seem to start automatically on boot, even though I've enabled the service via systemctl. I'll test it further (maybe it was a short-term problem), but at least I know how to get the console working!

Thanks again!

[pepedog]

No .service on the end? I wonder if that is needed?
BTW, files are in /usr/lib/systemd/system

[tawalker]

I still need to check whether the console service is now autostarting on boot (thanks for the lead, pepedog ), but while you're waiting for that: here's a short video to show the serial link is working fine...

[tedhale]

I am not really familiar with that distro, but assuming that is is like most linux distros, you need to have a line in

/etc/inittab

that looks like this

T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100

That should make the login prompt appear automatically on boot.

[pepedog]

I am not really familiar with that distro, but assuming that is is like most linux distros, you need to have a line in

/etc/inittab

that looks like this

T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100

That should make the login prompt appear automatically on boot.

Not any more, that was under init, replaced by systemd
To enable on boot

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systemctl enable getty@ttyAMA0.service

[tedhale]

Thanks for the correction. Once again showing what an old fart I am (though I did say I wasn't familiar with the distro.)
A lot of my experience is with Debian and Ubuntu, so Raspbian seemed the best choice for me to start with.
It still amazes me how quickly the number of choices is growing for the RasPi.

[pepedog]

Old fart? I'm not far off 59
Truthfully I am really getting attached to systemd

[tawalker]

Odd thing: I can start the service with

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# systemctl start getty@ttyAMA0

and the serial console runs fine, but if I run

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# systemctl enable getty@ttyAMA0.service

and reboot the Pi, the console service doesn't respond when I try and log onto it from a connected machine. (The systemctl "status" seems to say the service is enabled, but it doesn't appear to have auto-started.)

If I can access the Pi another way (SSH, keyboard/display), I can start the service manually, but that rather defeats the object of having the serial console (i.e. for accessing the Pi if other methods aren't available). Any ideas why the service may be enabled, but isn't autostarting?

Many thanks for your suggestions so far

[tawalker]

Truthfully I am really getting attached to systemd

I'm starting to grow used to systemd, mainly as I've had to - I did a fresh installation of the "hard-float" Arch/ARM on the Pi, and it came with systemd all set up, so I figured I'd roll with it

Mind you, I'm now in the weird situation of having two machines running Arch (the Pi, and my Eee 701SD netbook (x86)), where the Pi is set up with systemd and the Eee is still on initscripts. I'd like to switch the Eee over to systemd too, not least because the boot process keeps telling me the lines in rc.conf are deprecated

Thankfully the Arch wiki has a thorough guide on migration to systemd, but it looks a bit like open-heart surgery...

[greyfrog]

Hi tawalker and others,
I had the same problem with no serial console in Arch, then having it not start on boot. Your conversation got me going in the right direction to fix it. So I should post it here for everybody else:
First, 115200 seems to be too fast (at least in my case with an Adafruit serial to usb connector). Change it to 38400 in /boot/cmdline.txt :

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sdhci-bcm2708.enable_llm=1 sdhci-bcm2708.sync_after_dma=0 smsc95xx.turbo_mode=N dwc_otg.lpm_enable=0 console=ttyAMA0,38400 kgdboc=ttyAMA0,38400 console=tty1 loglevel=6 root=/dev/mmcblk0p2 init=/bin/systemd rootfstype=ext4 rootwait

Second, systemctl enable is not creating the proper service link:

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ln -s /usr/lib/systemd/system/getty@.service /etc/systemd/system/getty.target.wants/getty@ttyAMA0.service

Third, on the other end, minicom gave me no end of problems. gtkterm works better. screen works best:

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screen /dev/ttyUSB0 38400

The serial console is now working like I expect.
I just got my Pi yesterday. I'm not sure what I'm going to do with it yet, other than play around, but I'm having fun so far. Thanks for getting me going in the right direction with your posts.

/greyfrog

[pepedog]

Greyfrog,
you can also cp the file instead of symlink, and edit it.
It's not a hack but a way of customising

[SheepOnMeth]

Hello everyone,

I have the same issue. But I try all the solutions in this topic and I still have nothing on minicom, gtkterm or screen.

Systemd conf is good.

My cmdline.txt :

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smsc95xx.turbo_mode=N dwc_otg.lpm_enable=0 console=ttyAMA0,38400 kgdboc=ttyAMA0,38400 console=tty1 loglevel=6 root=/dev/mmcblk0p2 rootfstype=ext4 rootwait

I'm running Archlinux on my laptop.

[t3chm4g3]

Hi everyone,

The solution posted above by greyfrog is almost complete. What I'd like to add to it is this:
keep the baudrate at 115200 in cmdline.txt
and instead of spawning getty@.service spawn serial-getty@.service

With the above changes I can see the kernel log output at boot and login via serial console to my arch/raspberry pi.

[sdjf]

I am confused. Which end are you guys connecting to the GPIO pins and which end to your PCs?

And which tutorial or how-to or thread did you find the most helpful in figuring out the hardware setup?

[johnlane]

Very good, I had all this independently working at 38400 line speed. I have just changed it to 115200 using the suggestion from "t3chm4g3" to use serial-getty.service. The only desirable thing left that I do not have is to see the systemd startup messages on the serial port. Does anyone know how to achieve that ?

I believe this is due to data written to to /dev/console not being sent to the serial port.

It's funny how, as soon as you write a post about something, you find the solution When more than one console is listed in the kernel parameters, it is the the last one that gets assigned to /dev/console. So, to get /dev/console output on the serial port, ensure the console=ttyAMA0 entry comes after the console=tty1 entry in /boot/cmdline.txt.

My cmdline.txt is below for reference

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sdhci-bcm2708.enable_llm=1 sdhci-bcm2708.sync_after_dma=0 smsc95xx.turbo_mode=N dwc_otg.lpm_enable=0 console=tty1 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 loglevel=6 root=/dev/sda5 init=/bin/systemd rootfstype=ext4 rootwait

[pepedog]

Can you see in /boot/cmdline.txt there is a couple of console= entries?
Add another for your device

[sudar]

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systemctl enable getty@ttyAMA0.service

Can someone kindly tell me what is the equivalent of this command in Raspbian?

PS: My apologies if I am posting in the wrong place.

[spitfire]

Thanks everyone for the detailed suggestions.

After trying a lot of these variations, I found a simple solution for Redsleeve.

1. Start with the Redsleeve distribution raspi-v2-redsleeve-cli-0.3.img. The provided /boot/cmdline.txt already has ttyAMA0 listed as a console, just not the primary one:

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dwc_otg.lpm_enable=0 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait

2. Create a new file, /etc/init/ttyAMA0.conf:

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stop on runlevel [S016]
start on runlevel [2345]
instance /dev/ttyAMA0
respawn
pre-start exec /sbin/securetty ttyAMA0
exec /sbin/mingetty ttyAMA0

3. Then add one line to the existing /etc/securetty:

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echo 'ttyAMA0' >>/etc/securetty

Plug in the Adafruit cable, reboot the Raspberry Pi -- you can now log in via the HDMI / VGA console, and/or the serial console.

Interesting note -- if you connect via serial, leave the connection as-is then reboot, you will be able to view all the boot-time messages over the serial connection, which is handy.

Next I'm going to try to alter the Redsleeve image to add & alter the above files prior the install. If I can figure that out, I will share that too!